Recently, a network in which the packet forwarding functions and the path control or the like control functions for network equipment are isolated from each other, is attracting attention. In such network, the network equipment takes charge of the packet forwarding functions, while a separate controller provided on an outer side of and at a distance from the network equipment takes charge of the control functions. By so doing, it is possible to construct a network which is easy to control while being high in flexibility.
A technique known as OpenFlow to implement the above mentioned centralized control network has been proposed in Non-Patent Literatures 1, 2. The OpenFlow comprehends communication as an end-to-end flow and manages path control, recovery from malfunctions, load balancing and optimization from one flow to another. An OpenFlow switch, shown as the specification in Non-Patent Literature 2, includes a secure channel over which the switch may communicate with an OpenFlow controller, and operates in accordance with a flow table which may be supplemented or modified that is instructed as necessary from the OpenFlow controller. In the flow table, a set of match conditions (Match Fields) for matching against a packet header, the flow statistics information (Counters) and a set of instructions that define the processing contents (Instructions) are defined from one flow to another. Reference may be made to ‘5.2 Flow Table’ of Non-Patent Literature 2.
On reception of a packet, the OpenFlow switch retrieves the flow table to search for an entry having the match condition conforming to the header information of the received packet. See ‘5.3 Matching’ of Non-Patent Literature 2. If, as a result of the search, the entry matching the received packet is found, the OpenFlow switch updates the flow statistics information (Counters), and executes the processing contents stated in an instruction field of the matching entry, such as transmitting the packet out a specified port or flooding/dropping the packet. If conversely no entry matching the received packet is found, the OpenFlow switch sends an entry setting request to the OpenFlow Controller over the secure channel, by way of a request for having the control information sent to it in order for the switch to process the received packet (Packet-In message). The OpenFlow switch receives the flow entry, in which processing contents are stated, and accordingly updates its flow table. In this manner, the OpenFlow switch forwards the packet, using the entry stored in the flow table as the control information.
In Non-Patent Literature 3, there is disclosed a system in which no network for control is provided and a control channel is provided in a network for data transmitted between switches. This system is termed below an ‘in-band control system’, see e.g., Non-Patent Literature 3.    Non-Patent Literature 1:    Nick McKeown and seven others: “OpenFlow: Enabling Innovation in Campus Networks”, [online], [searched on January 9, Heisei25 (2013)], Internet <URL:http://www.openflow.org/documents/openflow-wp-latest.pdf>    Non-Patent Literature 2:    “Open Flow Switch specification”, Version 1.3.1 (Wire Protocol OxO4), [online], [searched on January 9, Heisei25 (2013)], Internet <URL:https://www.opennetworking.org/images/stories/downloads/specification/openflow-spec-v1.3.1.pdf>    Non-Patent Literature 3:    ‘A study on the automatic construction mechanism of control network in OpenFlow-based network’, by Toshio Koide and Hideyuki Shimonishi, Shingaku-Gihou, Society of Electronic Information Communication, a body corporate, NS2009-165 (2010-3), Vol. 109, No. 448, pp. 19-24, March 2010